1. Field of the Invention
This invention relates to a process for preparing EVA foam products, such as an EVA insole that has an open cell structure.
2. Description of the Related Art
Conventionally, EVA (ethylene vinyl acetate copolymer) foam is prepared by completely filling a preheated mold with an EVA foamable composition, which usually contains EVA resin, a foaming agent, and a cross-linking agent, applying pressure to the composition and allowing foaming and curing of the composition to take place (i.e. decomposition of the foaming agent and the cross-linking agent), and subsequently releasing the pressure to allow the EVA composition to simultaneously expand and cure. The EVA foam made from the conventional process has a cellular structure that contains cells which are mostly closed cells (e.g. 90% closed cells and 10% open cells).
Uniformity of the structure of the EVA foam is significantly affected by a balance of the decomposition of the blowing agent (a foaming process) to the decomposition of the cross-linking agent (a cross-linking process), which, in turn, is significantly affected by the uniformity of the foaming temperature in the mold.
FIG. 1 illustrates a foamable material 20 placed in a mold 10 and expanding in the latter according to a conventional foaming process for the production of a foam material. The mold 10 includes upper and lower mold halves 101, 102 which are heated to a temperature at which foaming and curing of the foamable material 20 take place. Since the foamable material 20 is laid on and is in direct contact with the lower mold half 102, different rates of foaming and curing for upper and lower portions of the foamable material 20 occur, thereby resulting in non-uniformity of the structure of the thus-formed foam material.
The object of the present invention is to provide a process for preparing an EVA foam that has a uniform structure which contains a relatively high ratio of open cells to closed cells as compared to the prior art. The process includes a series of foaming stages with different foaming temperatures for upper and lower mold halves of a mold employed in the process of this invention so as to obtain a uniform structure of the EVA foam in a commercially viable manner.
According to the present invention, there is provided a process for preparing EVA foam. The process comprises the steps of: kneading a mixture of an EVA resin, a foaming agent, a cross-linking agent, a lubricant, and a filler to form a kneaded composition; preparing a mold that has upper and lower mold halves which confine a mold cavity therebetween; placing an amount of the kneaded composition in the mold such that the volume ratio of the kneaded composition to the mold cavity is about 0.15 to 0.5 and such that the kneaded composition is laid on the lower mold half; heating the upper and lower mold halves in a manner that the temperatures of the upper and lower mold halves are not less than a reaction temperature, at which foaming and curing of the kneaded composition take place, so as to conduct a first stage of foaming and curing of the kneaded composition and so as to permit the kneaded composition to expand in the mold cavity from the lower mold half toward the upper mold half; and raising the temperature of the upper mold half to be higher than that of the lower mold half when the kneaded composition expands to a volume such that the volume ratio of the kneaded composition to the mold cavity is about 0.9 to 0.95 so as to conduct a second stage of foaming and curing of the kneaded composition.